Make join and meet symmetric with strict_optional (#18227)

Fixes #18199 

Updated `join.py` to return `AnyType(TypeOfAny.special_form)` for both
`UnboundType` and `AnyType`.

Updated `meet.py` to return `UninhabitedType()` when visiting a
`UnboundType` as a `NoneType` when `state.strict_optional` is true.

mypy/join.py

@@ -317,7 +317,7 @@ def visit_none_type(self, t: NoneType) -> ProperType:
         if state.strict_optional:
             if isinstance(self.s, (NoneType, UninhabitedType)):
                 return t
-            elif isinstance(self.s, UnboundType):
+            elif isinstance(self.s, (UnboundType, AnyType)):
                 return AnyType(TypeOfAny.special_form)
             else:
                 return mypy.typeops.make_simplified_union([self.s, t])

mypy/meet.py

@@ -691,7 +691,7 @@ def __init__(self, s: ProperType) -> None:
     def visit_unbound_type(self, t: UnboundType) -> ProperType:
         if isinstance(self.s, NoneType):
             if state.strict_optional:
-                return AnyType(TypeOfAny.special_form)
+                return UninhabitedType()
             else:
                 return self.s
         elif isinstance(self.s, UninhabitedType):

mypy/test/testtypes.py

@@ -806,8 +806,7 @@ def test_any_type(self) -> None:
             self.fx.anyt,
             self.fx.a,
             self.fx.o,
-            # TODO: fix this is not currently symmetric
-            # NoneType(),
+            NoneType(),
             UnboundType("x"),
             self.fx.t,
             self.tuple(),
@@ -1177,8 +1176,7 @@ def test_none(self) -> None:
             self.assert_meet(self.fx.o, NoneType(), NoneType())
             for t in [
                 self.fx.a,
-                # TODO: fix this is not currently symmetric
-                # UnboundType("x"),
+                UnboundType("x"),
                 self.fx.t,
                 self.tuple(),
                 self.callable(self.fx.a, self.fx.b),